As someone who first switched to lemmy, and then quickly switched to kbin due to rampant de-federation in the lemmy world, I say I just first heard about lemmy.
But, Kbin is much more modern, and spectating the changes done in the last days alone, it moves fast and attracts many developers while all lemmy 0.18 did was breaking federation with kbin. I can fully recommend the switch to Kbin, its that good.
There is also the issue of Lemmy being developed by a group of genocide denying tankies.
About Tildes, it seems to be more of a clone of Digg in the old days.
minnieo